i dont know, m using avast AV and so do most of my members, i have attached a screenshot here that contains the warning that pops up everytime someone surfs through my website. I have noticed that very few of the web pages dont give the warning but most of them do.

interesting indeed,i never sen that before. Double check all your /root .php files codes.
Also,i wonder if there's any new exploit using signatures ect.
I did went to your site using IE couple of times and only happened once. I got a pop up running a fake antivirus scanner trying to make me download a .exe file.
Now..also it might be a "BHO" somewhere. I wish i could of been more helpful.

it was an XSS vulnerability because of that particuler theme. I enever thought about that because i thought myBB is quite secured, IT IS. But just the problem in theme designed which allowed html entities.